I have a 3 week old baby, so face the same dilema. My mother flew up to visit him last week.
I have decided that even though it is annoying that my mother holds back from visiting me, but wants access to the baby, it is going to be the most effective way for me to build up a relationship with my mother, and maybe even chip away at her cult ideologies.
What I am making clear is that I will only be allowing access if her religious ideas are not spoken about in front of my child. Luckily my wife is not disfellowshipped so is able to deliver such messages better than I can who would end up in a bitter fight about it. Maybe you could write to your parents outlining that are very happy that they want to have a relationship with your daugther, but that you believe the Watchtower belief system to be damaging, and you will not tolerate them discussing their beliefs with her.
The other issue is that your daughter may pick up on the bad vibes with your parents. Let your parents know that shunning is unchristian and unhealthy for your child. If they are going to visit they must treat you as a normal family would.
The great thing with your situation now is that it is you that has control. Previously your parents thought they did. They could treat you like dirt and shun you. Now you have something they want. You are able to set the ground rules.
Regarding your daughter not having a proper relationship with her, as she gets older you will be able to blame the religion, so that she is not too hurt by the grand parents behaviour. Tell her that she is loved by her grandparents, but that they belong to a horrible religion that does not let them be very nice to people that do not go to their church. It will be a good foundation for you to educate her how some religions can be very harmful and what to look out for to avoid ever becoming involved is such organizations.
I have a 15 year old step daughter that my mother phones from time to time. I make sure to ask my daughter what was discussed each time in order to highlight the flaws of any damaging comments. I feel this is better that forcing total avoidance, as the best way to avoid cults is through education. My daughter is now able to identify false reasoning, and these days will come to me saying "can you believe she said this or that".